In hydraulic-elastomeric mounts such as used for mounting an internal combustion engine in a motor vehicle, a hydraulic damping decoupler can be employed to eliminate undesirable hydraulic damping at certain small vibratory amplitudes and low frequencies and thereby better isolate the vibrations of the engine under these conditions. Such decouplers are normally reciprocating devices which operate to effect alternating volume change in the two orifice connected chambers of the mount so as to not force flow through the orifice and thereby eliminate damping in this mode. While the performance of such decouplers has proven generally satisfactory, it has been found that the abrupt volume changes that they produce because of such reciprocation are in the form of a step function and can be sensed in the vehicle both in the undamped mode and as a distinct transition between this mode and the damped mode.